This invention pertains to catalytic hydroconversion of petroleum feedstocks containing preasphaltenes to produce lower-boiling hydrocarbon liquid products. It pertains particularly to a catalytic hydroconversion process in which the residual fraction boiling above about 975.degree. F. is treated with an acid to precipitate out preasphaltenes, metals and nitrogen compounds, prior to recycling the supernatant stream to the reaction zone.
It is known that recycle of residual oil fractions can increase the percentage conversion achieved in catalytic hydrogenation operations, such as for H-Oil.TM. process operations on heavy petroleum feedstocks. Unfortunately, the residual material cannot be recycled to extinction in the reactor because it is necessary to eliminate from the system inorganic contaminants such as metals, sediment, and spent catalyst fines, and refractory organic materials such as polycyclic aromatics and carbon. It is desirable to provide a means for eliminating as much of this inorganic material as possible, and recycling as much as possible of the organic material to the reaction zone for further conversion to distillate products.
One process for removing preasphaltenes from heavy petroleum feedstocks is based on using a solvent precipitation step, wherein a light naphtha product fraction which is a poor solvent is added to the reacted separator bottoms material to precipitate preasphaltenes and solids. The separator overhead material which would then be substantially free of preasphaltenes and solids is passed to the fractionation step, and the bottoms material recycled to the catalytic reactor, so that the non-distillable material could be further cracked without building up high levels of solids and high viscosity organics in the reactor. U.S. Pat. No. 2,209,123 to Koelbel discloses purification of coal tar oils to remove undesirable asphalt, resin and pitch by treatment with a mixture of paraffin hydrocarbon and a dilute acid such as 20% sulfuric acid. Also, U.S. Pat. No. 3,085,061 to Metrailer, discloses a shale oil refining process using anhydrous hydrogen chloride to treat the oil and return a sludge stream to a coker or reactor. However, these processes recycle to the reactor the heavy bottoms material containing inorganic components.